1. Field of the Invention
The present invention relates to a dissolved oxygen removal method using an activated carbon fiber and an apparatus thereof for preventing corrosion of metal due to dissolved oxygen contained in water used in a steam generator and a cooling water system, and in particular, to an improved dissolved oxygen removal method using an activated carbon fiber and an apparatus thereof which are capable of significantly enhancing removal performance of dissolved oxygen contained in water when impregnating metals on an activated carbon fiber based on the characteristics that an activated carbon fiber (ACF) has an excellent adsorption force for dissolved oxygen and a high surface area, so that the method and apparatus according to the present invention are effectively applicable for a water treatment system of a steam generator and a cooling water system.
2. Description of the Conventional Art
The following table 1 illustrates dissolved oxygen concentration contained in water at atmospheric pressure. As shown therein, a large amount of dissolved oxygen (8xcx9c10 ppm) is contained in water at room temperature when water is exposed to air.
Concentration of dissolved oxygen in water based on temperature
In the table, T represents temperature (xc2x0 C.), and D represents dissolved oxygen (ppm).
When the water in which dissolved oxygen is saturated is directly used for a steam generator, a corrosion may occur in a metal for thereby decreasing the life time of the system and causing operational problems. The corrosion of the metals due to the dissolved oxygen is composed of a cathodic reaction in which a reduction reaction of dissolved oxygen occurs and an anodic reaction in which an oxidation reaction occurs.
Corrosion reaction of Fe due to dissolved oxygen
i) Cathodic reaction: O2+4H++2H2O (acid solution)
xe2x80x83O2+2H2O+4exe2x86x924 OH+(neutral, alkali solution)
ii) Anodic reaction: Fexe2x86x92F+2+2e
Fexe2x80x22+2 OHxe2x88x92xe2x86x92Fe(OH)2(or FeOnHO2)
4 Fe(OH)2+O2+2H2Oxe2x86x924 Fe(OH)3(or Fe2O3nH2O)
2 Fe(OH3)xe2x86x92Fe2O3+H2O(high temperature)
The corrosion product of Fe is classified into three products. Among which, Fe(OH)2 is green in color and produced at a neighboring surface of the metal, Fe(OH)3 is grey in color and produced at an outer most portion, and Fe3O4 H2O are black color in and produced between the above-described two layers.
The metal corrosion in water due to the dissolved oxygen is proportional to the concentration of the dissolved oxygen and linearly increases. In the case that there is no dissolved oxygen, the corrosion rate of Fe is low (below 0.2 mpy), and when the water is saturated by the dissolved oxygen, the corrosion rate is increased by more than 100 times. When corrosion occurs, the process heat efficiency ratio is significantly decreased because the corrosion products are formed on the heat transfer surfaces, thereby decreasing the life time of the system.
Therefore, in order to maintain a corrosion prevention effect and a heat efficiency of a metal based on the dissolved oxygen, the concentration of the dissolved oxygen contained in water should be strictly limited. The following table 2 illustrates an example that the limits of the dissolved oxygen suggested by steam generator manufacturers. As shown therein, it is recommended to maintain the concentration of the dissolved oxygen below 10 ppb in the normal operation mode.
In the conventional dissolved oxygen removal method, the dissolved oxygen in water used for the steam generator is removed based on a mechanical degasifier and reducing agent (for example, hydrazine). However, the above-described conventional methods have the following disadvantages in view of the processing performance and cost.
i) Vacuum Deaerating Method
This method has been most widely used for removing the dissolved oxygen of make-up water in a steam generator of a nuclear power plant. The operational principle is to spray water into a vacuum tower and to decrease the pressure of the tower gas phase for thereby removing a non-condensable gas. The packed towers are preferably designed in more than two stages for thereby increasing the oxygen removing efficiency.
The dissolved oxygen removing efficiency is affected by the vacuum degree, the size of the packed tower, the water temperature at an entrance, etc. The packing material preferably has a large surface area per unit volume and then the size of the packed tower is determined. There should not be a by-product of an impurity from the packing material, and the vacuum degree should be maintained at a predetermined degree using a vacuum pump and a vapor ejector for thereby fully removing oxygen, nitrogen and CO2 from the packed tower. The water processed by the vacuum deaerating method contains a dissolved oxygen in a range of 30xcx9c40 ppb. Namely, it is impossible to perfectly remove the dissolved oxygen. The dissolved oxygen concentration is increased by air leakage at the sealed portions. In addition, a special sealing apparatus is required for maintaining the system at a predetermined vacuum degree. In addition, in order to maintain a vacuum state in the interior of the packed tower, an expensive apparatus is and needed high maintenance costs are incurred.
ii) Thermal Deaerating Method
The gas solubility in water is proportional to the partial pressure in a gas phase according to the Henry""s rule. Therefore, it is possible to remove the dissolved oxygen in water by decreasing the partial pressure in a gas phase. In addition, the gas solubility is decreased as the temperature of water is increased. The thermal deaerating method heats the water using the heating steam and to removes the dissolved gas by decreasing the partial pressure of a gas.
In this thermal deaerating method, the dissolved oxygen may be decreased below 7 ppb under the optimum operation of the thermal deaerator; however, it is impossible to apply the above-described thermal deaerating method to a system in which there is not a heating source such as steam and heater.
As one of the effective methods for removing the dissolved oxygen in water, there is a method using a reducing agent such as hydrazine. The oxygen removing chemical reaction of hydrazine in water is as follows. Since nitrogen gas and water molecular are produced as by-product of the reaction and do not effect the corrosion of the metal, the above-described method is widely used for removing the dissolved oxygen.
N2H4+O2xe2x86x92N2+2H2O
In this method, hydrazine 1 ppm per oxygen 1 ppm is consumed as a chemical agent. However, since the reaction is implemented at a relatively high temperature (above 80xc2x0 C.), it is very difficult to remove the dissolved oxygen at room temperature.
Accordingly, it is an object of the present invention to provide a dissolved oxygen removal method using an activated carbon fiber and an apparatus thereof which overcomes the aforementioned problems encountered in the conventional art.
It is another object of the present invention to provide a dissolved oxygen removal method using an activated carbon fiber and an apparatus thereof which are capable of extending the life time of the facility and decreasing the operational and maintenance costs by fully removing dissolved oxygen from water used for an steam generator or a cooling system for thereby minimizing corrosion of metal and effectively removing the dissolved oxygen in water by a metal supported activated carbon fiber or an activated carbon fiber.
In order to achieve the above objects, there is provided a dissolved oxygen removal method using an activated carbon fiber which includes the steps of a reducing agent injection step for injecting a reducing agent into the water, a reactor entrance dissolved oxygen measuring step for measuring a concentration of a dissolved oxygen contained in a water, a control or measurement step of water flow rate, an oxygen removal catalytic reaction step of dissolved oxygen and reducing agent in an activated carbon fiber reactor in which an activated carbon fiber catalyst is packed, and a flow control step of reducing agent in accordance with the concentration of the dissolved oxygen and the water flow rate at the entrance of the reactor.
In order to achieve the above objects, there is provided a dissolved oxygen removal apparatus using an activated carbon fiber which includes a water tank for storing water therein, a water pump connected with the water tank for pumping water, a reducing agent injection unit for injecting a reducing agent into water from the water tank, a water flow controller for supplying the water to a activated carbon fiber reactor, a reactor inlet dissolved oxygen measuring unit connected with the water flow controller for measuring dissolved oxygen, an activated carbon fiber catalyst reactor connected with the water pump, in which reactor an activated carbon fiber catalyst is packed, for performing a catalytic reaction between the dissolved oxygen and the reducing agent, a water quality monitoring unit for automatically recording the measured values of the dissolved oxygen measuring unit at the inlet and outlet of the activated carbon fiber catalyst reactor.
In the present invention, it is possible to implement an excellent dissolved oxygen removing effect based on the dissolved oxygen removing apparatus according to the present invention.
Additional advantages, objects and other features of the invention will be set forth in part in the description which is follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims as a result of the experiment compared to the conventional arts.